Images on electronic displays are derived from an array of small, picture elements known as pixels. In color displays, these pixels comprise three color elements that produce the primary colors, for example, red, blue and green (R, B and G). Usually arranged in rectangular arrays, these pixels can be characterized by a pixel pitch, P, a quantity that measures the spacing of pixels in one direction. A typical cathode-ray tube (CRT) display used for computer applications has a pixel pitch have a pixel array width:height ratio of 4:3. Typical, standardized arrays in computer displays are composed of 640.times.480 (VGA), 800.times.600 pixels (SVGA), or 1024.times.780 pixels (XGA).
Large displays can be constructed of a plurality of adjacent tiles, with each having a single pixel or an array thereof. Such assembled tiled displays contain visually disturbing seams, resulting from the gaps between adjacent pixels on adjacent tiles. Such seams may incorporate interconnect, adhesives, seals, mechanical alignment means and other components resulting in optically visible discontinuities in displayed images. Some of these structures are described in the aforementioned U.S. Pat. No. 5,661,531. As a consequence, the image portrayed on seamed displays appears segmented and disjointed. Therefore, it is desirable to fabricate tiled, flat-panel displays which do not have noticeable or perceptible seams under the intended viewing conditions.
The pixel pitch in electronic displays must be set so that a continuous image is produced when the display is viewed at distances greater than the minimum viewing distance. For example, with a pixel pitch of P=0.3 mm. the minimum viewing distance is on the order of 1 m. Even though the minimum viewing distance increases in proportion to the pixel pitch, it still limits the pixel pitch for most computer and consumer displays. Since space for the tiling functions must be provided in spaces smaller in size compared to the pixel pitch, it is difficult to develop structures and methods for constructing tiled displays.
Flat-panel displays (FPDs) provide the best choice for constructing "seamless", tiled screens. Flat-panel displays include back lighted and self-lighted displays. Liquid crystal displays (LCDs) are the most common back lighted displays. Flat-panel displays depend on the micro fabrication of key components that carry the pixel patterns. Such micro fabrication techniques, however, are not viable for very large displays, generally greater than 20 inches diagonal, due to the fact that the manufacturing yield declines rapidly with increasing area of the display. Therefore, the inventors have determined that tiles with arrays of pixels can be micro fabricated and then assembled together to form a larger electronic display.
The present invention provides unique designs and methods for achieving such large, seamless, tiled panels for color or gray-scale displays. This invention particularly focuses on displays of the transparent, light valve type. In such displays, light from a uniform, backlight source is transmitted through the display assembly and directly viewed from the front side of the display. The light valves control the amount of primary light rays transmitted through each of the color elements in the pixels. The viewer's eyes merge the primary light from the pixels to form a continuous image at a sufficient viewing distance.
Because of a number of secondary processes, low-level light emanates from the phenomena include reflection and light guiding, all of which must be kept to a minimum in order to achieve sufficient brightness and contrast. The spaces between pixels on the same tile and the spaces between pixels on adjacent tiles have different structures. Consequently, the presence of seams between the pixels at the edge of the tiles will affect both primary and secondary light rays, thus making the construction of seamless, tiled displays more difficult.
In addition to the optical and electronic correction means the inventors have identified three design principles in assembling large-size, seamless, flat panels that may be viewed as though they were single monolithic displays:
a) the pixel pitch on the view plane for the tiles must be matched to that of the pixel pitch on the view plane between the tiles within a critical set of tolerances;
b) the primary light paths through the light valves must not be substantially affected by the presence of the seam or any other structures or components used in the tile assembly; and
c) the inter-pixel gaps must be designed so that intratile and inter-tile pixel gaps, which have different physical structures, present approximately the same visual appearance to the viewer under both transmitted and reflected light.
This has largely been accomplished by applying the technology disclosed in the U.S. Pat. No. 5,661,531 in fabricated, tiled AMLCD functional models. However, design improvements are possible to increase the manufacturing yield and the optical performance of the tiled displays from their component tile parts. This invention focuses on preferred assemblies of the tiles into robust laminates between glass cover plates and back plates.
Tiled FPDs require a high degree of location precision and alignment in all three dimensions, X, Y, and Z, to appear monolithically, optically continuous, pixel to pixel, across seams between neighboring tiles. The means to achieve AMLCD tiled FPDs in this invention requires referencing the tiles along the Z dimension with adhesive films of preferred thickness. The tiles are spaced by the films between continuous, optically flat, cover and back plates, having matched indices of refraction. At the same time, the horizontal and vertical dimensions of the FPD tiles are maintained, locating the pixels across the seams with continuity in pitch and parallelism, by optical means and by the use of these preferred polymer film spacers. Compliant, adhesive films in the range of 25 to 250 microns in thickness, and optimized in compliance, are used to bond the tiles to the cover and back plates without inadvertently stressing the tiles and deforming the cell gap. The air is controllably purged at the meniscus of the adhesive interface by using an assembly machine which actually bends the adhesive coated glass cover and back plates to a critical radius while attaching the adhesive to the tiles. The glass back or cover plates may be a standard such as 1737 commonly used in the AMLCD industry. Thickness standards are 0.5 mm, 0.7 mm and 1.1 mm. For these tiling assemblies any of these glasses may be used. The thinner glasses allow smaller radius to be used in the adhesive extrusion process. This makes a robust laminated assembly with continuity of refractive index, and well matched thermal expansion to the standard glass tile materials.
Previously used apparatus for forming the laminated composite have applied the adhesive in liquid form in a puddle followed by squeezing to meet an approximate thickness specification. This was a slow and expensive process, due to the need for highly sophisticated precision machines, in conjunction with aligning or fiducial marks, to control the X, Y, and Z dimensions over the large areas of the entire FPD. In this current design and process, however, locations are more precise by pixel to pixel location at the tile edges without the loss of tolerance customary in fiducial optical location systems. The gain in tolerance may be applied to improving the aperture ratio of the pixels and/or line resolution of the tiled display. Alternatively, for the same resolution and aperture ratio, a wider seal at the tile edges may be used to increase the yield of the tiles. The process yield and speed of the assembly processes are significantly increased over the methods of the prior art.